electrostatic lens

The electrostatic lens for SEM is composed of two electrodes and accelerates or decelerates an electron beam.
In a field-emission gun (FEG), the extraction electrode and acceleration electrode which are placed below the cathode constitute an electrostatic lens. For the extraction electrode, a voltage of 4 to 5 kV is applied. When a voltage higher than that for the extraction electrode is applied to the acceleration electrode, the electrostatic lens acts to accelerate the electron beam emitted from the cathode. To the contrary, when a voltage lower than that for the extraction electrode is applied to the acceleration electrode, the electron beam from the cathode is decelerated. For secondary electron imaging of a conductive specimen and elemental analysis by EDS, the electrostatic lens is used to accelerate the electron beam up to 10 to 15 kV. For imaging of a nonconductive specimen, the lens is used to decelerate the electron beam down to 1 kV or lower.
Recently for the high-resolution SEM equipped with an FEG, an objective lens has been developed for practical use to obtain a high-resolution image by reducing the volume of the diffusion region. In this objective lens, an electron beam at several to 10 kV is decelerated down to an extremely low voltage of 1 kV or lower just before a specimen. This objective lens is composed of an electromagnetic-and-electrostatic field composite lens. The electromagnetic lens demagnifies the electron probe and an electrostatic lens decelerates the electron probe, a short focal length being created as total. An electron probe with a small probe size and low acceleration enables to acquire a high-resolution SEM image.